Diddy -- WTEff? Machine Gun Kelly Drinks Enemy Vodka With 50 Cent

Bad Boy rapper Machine Gun Kelly just gave Diddy a real slap in the face by fraternizing with the enemy, 50 Cent, and pounding Fif’s rival vodka like a poster boy for the stuff.

MGK crashed 50’s Pool After Dark party last weekend in Atlantic City. Not a big deal ... except it was an Effen vodka event and MGK is signed to Diddy’s Bad Boy Records where Ciroc rules supreme.

We’re told all Bad Boy artists are supposed to plug Diddy’s vodka … it comes with the territory. MGK even bathes in the stuff in his 'LTFU' video.

MGK screwed up by getting up onstage in front of a giant Effen banner with 50 at the party.

50 and Diddy have a storied beef and 50's repeatedly played off the rivalry when plugging Effen ... going as far as using the hashtag #nomorepuffyjuice.

The hashtag's made its way to t-shirts ... that people were wearing … inside the booth… where MGK was taking back the rival spirit ... total WTEff?